In Sec. 4.9 we defined hypersonic flow as that flow where the Mach number is 5 or greater. Wind tunnels with a test section Mach number of 5 or greater are called hypersonic wind tunnels. From Eq. (4.88), the exit-to-throat area ratio for supersonic exit Mach numbers increases as the exit Mach number increases. For hypersonic Mach numbers, the exit-to-throat ratio becomes extremely large, so hypersonic wind tunnels are designed with long, high-expansion-ratio nozzles. In this and the following problems, let us examine some special characteristics of hypersonic wind tunnels. Assume we wish to design a Mach 10 hypersonic wind tunnel using air as the test medium. We want the static pressure and temperature in the test stream to be that for a standard altitude of 55 km. Calculate (a) the exit-to-throat area ratio, (b) the required reservoir pressure (in atm), and (c) the required reservoir temperature. Examine these results. What do they tell you about the special (and sometimes severe) operating requirements for a hypersonic wind tunnel?